1. Field of the Invention
The subject invention relates generally to a variable compression ratio engine in which the compression ratio in a cylinder for an internal combustion engine is adjusted while the engine is running, and more specifically toward an improved piston and connecting rod arrangement for dynamically varying the engine compression ratio.
2. Related Art
Gasoline engines have a limit on the maximum pressure that can be developed during the compression stroke. When the fuel/air mixture is subjected to pressure and temperature above a certain limit for a given period of time, it autoignites rather than burns. Maximum combustion efficiency occurs at maximum combustion pressures, but in the absence of compression-induced autoignition that can create undesirable noise and also do mechanical damage to the engine. When higher power outputs are desired for any given speed, more fuel and air must be delivered to the engine. To achieve greater fuel/air delivery, the intake manifold pressure is increased by an additional opening of a throttle plate or by the use of turbochargers or superchargers, which also increase the engine inlet pressures. For engines already operating at peak efficiency/maximum pressure, however, the added inlet mixtures created by turbochargers and superchargers would over compress the combustion pressures, thereby resulting in autoignition, often called knock due to the accompanying sound produced. If additional power is desired when the engine is already operating with combustion pressures near the knock limit, the ignition spark timing must be retarded from the point of best efficiency. This ignition timing retard results in a loss of engine operating efficiency and also an increase of combustion heat transferred to the engine. Thus, a dilemma exists: the engine designer must choose one compression ratio for all modes. A high compression ratio will result in optimal fuel efficiency at light load operation, but at high load operation, the ignition spark must be retarded to avoid autoignition. This results in an efficiency reduction at high load, reduced power output, and increased combustion heat transfer to the engine. A lower compression ratio, in turn, results in a loss of engine efficiency during light load operation, which is typically a majority of the operating cycle.
To avoid this undesirable dilemma, the prior art has taught the concept of dynamically reducing an engine compression ratio whenever a turbocharger or supercharger is activated to satisfy temporary needs for massive power increases. Thus, using variable compression ratio technology, the compression ratio of an internal combustion engine can be set at maximum, peak pressures in non-turbo/super charged modes to increase fuel efficiency while the engine is operating under light loads. However, in the occasional instances when high load demands are placed upon the engine, such as during heavy acceleration and hill climbing, the compression ratio can be lowered, on the fly, to accommodate an increase in the inlet pressure caused by activation of a turbocharger or supercharger. In all instances, compression-induced knock is avoided, and maximum engine efficiencies are maintained.
Various attempts to accomplish dynamic variable compression ratios in an internal combustion engine have been proposed. For example, the automobile company SAAB introduced a variable compression ratio engine at the Geneva Motor Show in the year 2000. The SAAB design consisted of a monoblock cylinder head and a separate crankshaft/crankcase assembly. The monoblock head was connected by a pivot to the crankshaft/crankcase assembly, so that a small (e.g., 4°) relative movement was permitted, which movement was controlled by a hydraulic actuator. The SAAB mechanism enabled the distance between the crankshaft center line and the cylinder head to be varied.
Other attempts to accomplish dynamic variable compression ratios have included an effective lengthening/shortening of the connecting rod, which joins the reciprocating piston to a rotating crankshaft. Among the myriad designs which favor adjusting the length of a connecting rod, some are proposed in which an eccentric wristpin connection is provided at the articulating joint between the small end of the connecting rod and the piston. Examples of eccentric wristpin constructions may be found in U.S. Pat. No. 2,427,668 to Gill, issued Sep. 23, 1947, and U.S. Pat. No. 4,687,348 to Naruoka et al., granted Aug. 18, 1987, and also U.S. Pat. No. 4,864,975 to Hasegawa, granted Sep. 12, 1989.
A particular shortcoming in all prior art attempts to extend or shorten the length of the connecting rod through an eccentric bushing at the small (upper) end of arises from the consistent use a small offset distance between the piston pin axis and the center of the eccentric bushing's outer diameter (i.e., the center of the eccentric bearing rotational axis). With the small offset dimension, the prior art eccentric bushing must rotate through a very large angle to achieve the desired change in connecting rod length. FIG. 20 is illustrative of prior art designs, and suggests a total rotational angle of about 160° to achieve a complete height change. This large rotation angle is considered advantageous because it allows the prior art eccentric bushing to be made relatively smaller in diameter and, because the rest points are very near top and bottom dead center positions, there is a favorable reduction in the load carried by the latching features during normal engine operation (i.e., when the eccentric bushing is locked in one position). The disadvantage of this prior art configuration manifests during the switching of rod assembly length. As the large (crank) end of the connecting rod moves sideways with crankshaft rotation, it causes a rotation of the connecting rod that generates a torque at the eccentric bushing and eventually the eccentric bushing rotates enough for the connecting rod's axial force, working on the (now larger) effective moment arm, to accelerate rotation of the eccentric bushing. However, by this time the axial load on the connecting rod has increased to a substantial level and as the rod's length changes, a very large amount of available energy goes into the rotation of the bushing. The latching feature at the far end of the travel must then absorb all of this kinetic energy and may be damaged and make noise from the impact.
Accordingly, there is a need for an improved switching mechanism that will extend the useful service life of the latching feature.